1. Technical Field
The present invention relates to a liquid crystal device, an active matrix substrate, and an electronic apparatus.
2. Related Art
Reflective type liquid crystal devices are mounted on electronic apparatuses, such as cellular phone, notebook computer, and reflective type projector. The reflective type liquid crystal device has a structure in which a liquid crystal layer is interposed between a glass substrate or a silicon substrate provided with data lines, scan lines, switching elements such as transistors, storage capacitors, and reflective type pixel electrodes made of aluminum and a glass substrate provided with a counter electrode which is a transparent electrode. Since the pixel electrodes are reflective type, it is possible to dispose the switching elements, such as transistors under the pixel electrodes and to increase resolution while avoiding the decrease of an aperture ratio. That is, it is relatively easy to achieve both high resolution and high brightness.
However, in the case of using a pixel circuit based on an analog system which maintains a constant pixel voltage by the use of a storage capacitor, there is a problem in that brightness and contrast of a display image may easily change because a voltage of the storage capacitor becomes lowered as time passes.
In order to solve this problem, JP-A-8-26170 suggests a liquid crystal device in which bits of memory cells are disposed under respective reflective type pixel electrodes of pixels for every pixel. In the liquid crystal device having pixels each provided with a bit of memory cell, image signals from the data lines are latched by the memory cells and the latched signals are applied to the liquid crystal layer of the pixels. The memory cell maintains a previously input signal until a new signal is input. Accordingly, it is possible to effectively perform display switching operation in a simple manner, in which a still image is saved in a memory first, a different image is then displayed, and finally the image saved in the memory is displayed again. Further, it is also possible to suppress degradation of display quality attributable to crosstalk by digitizing the pixel voltage.
JP-A-5-303077 discloses an effective technique in which a voltage polarity applied to a liquid crystal is periodically inverted in order to prevent image sticking (deterioration of a display image attributable to a phenomenon in which liquid crystal molecules are aligned in a certain direction) from occurring when a direct current voltage is applied to a liquid crystal.
A circuit structure of the liquid crystal device having pixels each provided with a memory cell is disclosed in JP-A-2005-148453 and JP-A-2005-25048. The techniques disclosed in JP-A-2005-148453 and JP-A-2005-25048 are common in the point that voltage polarities applied to one electrode and a counter electrode (common electrode) of a liquid crystal are periodically inverted. According to the technique disclosed in JP-A-2005-148453, decision that which of complementary signals which can be obtained from a static random access memory (SRAM) is supplied to the liquid crystal is made by switching on/off of a transistor. According to the technique disclosed in JP-A-2005-25'048, since an offset voltage generated when a voltage polarity applied to a liquid crystal is inverted causes image sticking, an offset voltage applied to the counter electrode (common electrode) adjusted in a manner such that response waveforms obtained by the output from an optical sensor become equal to each other for every field.
As for a liquid crystal device, there is known a liquid crystal device in which alignment of liquid crystal molecules is controlled by applying an electric field in a direction parallel to a surface of a substrate to a liquid crystal layer. With reference to JP-A-2001-337339, the liquid crystal device is called an In-Plane Switching (IPS) system or a Fringe-Field Switching (FFS) system liquid crystal device depending on the shape of electrodes by which an electric field applied to a liquid crystal is generated. In the lateral electric field system liquid crystal device, light transmittance is controlled by rotating horizontally aligned liquid crystal molecules in a lateral direction. Since liquid crystal molecules are not tilted to a vertical direction at an angle, brightness and color variation attributable to a viewing angle are small. Accordingly, the lateral electric field system liquid crystal device can be used for applications needing a wide viewing angle and a high quality chromic characteristic.
In order to prevent sticking of a liquid crystal from occurring, a direct current voltage must not be applied to a liquid crystal for a long time. FIGS. 13A and 13B are views illustrating the operation of preventing sticking from occurring in the liquid crystal device. FIG. 13A shows an operation state in which a voltage is applied to a liquid crystal and FIG. 13B shows an operation state in which a voltage is not applied to the liquid crystal. FIGS. 13A and 13B relate to a twisted nematic liquid crystal device (TN LCD) in which an electric field is applied to a liquid crystal layer in a direction perpendicular to a surface of a substrate.
As shown in FIG. 13A, in the case in which a voltage is applied to a liquid crystal 400, a voltage polarity applied to the liquid crystal 400 is periodically inverted in order to prevent image sticking from occurring. That is, a polarity of a voltage applied to each of terminals X1 and X2 in this figure is periodically switched. Further, the liquid crystal 400 have a lower electrode Lp and an upper electrode (common electrode) LCcom on both sides thereof.
As shown in FIG. 13B, in order to prevent image sticking from occurring in the case in which a voltage is not applied to the liquid crystal 400, the lower electrode Lp and the upper electrode (common electrode) LCcom must have the same potential, which is achieved by causing a short-circuit between the lower electrode Lp and the upper electrode LCcom. To this end, it is important that a direct current offset is not generated. In FIG. 13B, for convenience's sake, it is possible to make the electrodes of the liquid crystal be short-circuited using a switch SW1. However, in practical, the shore-circuited state of the electrodes of the liquid crystal 400 is accomplished by applying the same voltage to the electrodes.
However, in the liquid crystal device having pixels each with a memory circuit, as schematically shovel in FIGS. 13A and 13B, it is difficult to realize ideal operation (ideal polarity inverting operation and electrode short-circuiting operation for preventing image sticking from occurring).
FIGS. 14A to 14C relate to a liquid crystal device including pixels each with a memory circuit. FIGS. 14A to 14C are views for explaining problems encountered when inverting voltages applied to both electrodes of the liquid crystal.
As for a method of inverting polarities of voltages applied to both electrodes of a liquid crystal, as shown in FIG. 14A, there is known a method in which a voltage Vcom of the counter electrode (common electrode) is fixed and a voltage Vp applied to the lower electrode Lp is inverted. Further, as shown In FIG. 14B, there is known an alternative method in which both voltages Vp and Vcom applied to the lower electrode Lp and the common electrode LCcom, respectively, are simultaneously inverted in their polarities. In FIGS. 14A to 14C, the voltages applied to the liquid crystal are 5V and 0V.
It is convenient to employ the method shown in FIG. 14A because it does not need to change a potential (Vcom=0V) of the counter electrode (common electrode) LCcom. However, a negative power source must be used for this method because it needs to change the voltage (Vp) of the lower electrode Lp relative to the potential Vcom. Since it is impractical to drive the memory circuits provided to the pixels using a negative power source, the method shown in FIG. 14A cannot be used in the liquid crystal device with memory circuits.
For such a reason, there is no other choice but to use a method of simultaneously changing voltages Tip and Vcom applied to the lower electrode Lp and the common electrode LCcom as shown in FIG. 14B. However, this method has a problem in that the liquid crystal layer interposed between the substrates acts as a capacitor as a whole and thus the voltage changing is slow because the counter electrode (common electrode) LCcom is a common electrode shared by all pixels.
That is, as shown in FIG. 14C, load of the lower electrode Lp is light because one lower electrode Lp corresponds to only one pixel. Thus, at a time t1 when voltages applied to both electrodes of a liquid crystal are inverted, the voltage Vp applied to the lower electrode Lp can be rapidly changed. On the other hand, the voltage applied to the counter electrode (common electrode) LCcom is changed after a lapse of transition time T1 (from t1 to t2) because the counter electrode has heavy load as shown in FIG. 14C. As a result, in the transition time T1, the voltage applied to the liquid crystal gradually varies as time passes. The change of light transmittance which is attributable to the voltage change is readily caught by the eye and thus flickers (complementary flickers) occur.
In order to control voltage inverting operation shown in FIG. 14B, the voltage Vp and the voltage Vcom must be individually controlled by different control circuits, respectively and thus it is natural that the circuit structure is complex.
FIGS. 15A and 15B are explanatory views for explaining the problem with the short-circuited state (the same potential state) of the electrodes of the liquid crystal in a liquid crystal device with pixel circuits each having a memory circuit. As shown in FIG. 15A, the electrodes Lp and LCcom of the liquid crystal 400 are applied with different ground potentials GND1 and GND2 from different circuits (wirings), respectively. However, as for the ground potentials GND1 and GND2 applied to the electrodes Lp and LCcom via the different circuits (wirings), since voltage levels therefore independently vary, there is a relative voltage difference between them.
Further, since the electrodes Lp and LCcom of the liquid crystal have two-dimensional broadening, the voltages Vp and Vcom of the electrodes vary over their entire areas and thus a direct current offset between both electrodes in each pixel may be generated.
Accordingly, as shown in FIG. 15B, it happens that a direct current offset voltage ΔV is generated between the electrodes opposing each other in each pixel of the liquid crystal 400. In FIG. 15B, Vgnd1 and Vgnd2 denote voltages applied to both the electrodes in each pixel of the liquid crystal, in which the voltages Vgnd1 and Vgnd2 are set considering local irregularity of voltage in planes of the electrodes. The direct current offset voltage ΔV leads to occurrence of image sticking.
In the liquid crystal device including pixels each provided with a memory circuit, it is difficult to achieve the perfect short-circuited state in which voltage inverting is performed in order to prevent image sticking from occurring while avoiding the flickers and without generating the direct current offset. Further, since the voltages applied to the electrodes Lp and LCcom of the liquid crystal must be individually controlled, the circuit structure is very complex.